The Roundship is the very first "heavy" ship made available to most factions. Although slow and somewhat unwieldly, the Roundship however can afford to carry more troops, and is roomier than oar-powered dromons and barques, meaning that it can fire off more missiles, despite having the same degree of firepower as a Barque. Thus at sea, the roundship is a highly destructive unit that can lay waste to an enemy galley fleet, and are fairly useful against enemy forts and castles, although they must be built in huge numbers in order for them to succeed, given the ability of castles to outrange them.

Roundships, however have several problems. The first is due to their need to be sufficiently strong and stable, you will be asked to commit considerable amounts of metal and timber to their construction. Only the Chinese and Japanese, with their know-how, can reduce their metal costs at the expense of added wood consumption. Also, as a heavy ship, roundships suffer from poor mobility, and so lack the ability to avoid fire ships head-on. A fleet of roundships must be screened at all times by fire ships or lighter galleys to prevent them from being literally toasted to perfection. Additionally, Roundships are not always available to everybody. Factions that use Dark Age Longships, with the exception of England, are constrained to use them, in no small part due to political turmoil.

Although large sailing ships have existed since the beginning of civilisation itself, sailing ships surprisingly were not that popular as a choice for combat, probably perhaps due to the lack of mobility that large wooden vessels would have had using primitive sails and rigging for navigation. However, they did serve their purpose in combat duty: a ship with a higher board and more deck space could hold more men on board, making them more deadly in battle as compared to the more narrow but faster galleys.

Factions fronting the Yellow Sea build roundship-type units earlier than Europeans, which are known for their exorbitant cost in timber but cost far less in metal.

The early medieval roundship was also known as Navis Oneraria, or "ship of burden". Featuring a flat-bottomed hulls and dual steering oars, it was commonly classified by carrying capacity, because of its importance in trade. For example, a roundship which could carry cargo weighing 300 metric tonnes would be called a 300-tonner.

In Europe, the common practice in war (and trade, because of pirates) was to escort roundships with galleys. The galleys could be used for scouting and skirmishing, while the roundship would serve as a floating "arsenal" or an enhanced weapon platform, preferably for launching missiles into the foe. The renowned workhorse of mediaeval empires, the cog, was a variant of roundship unique to Northern Europe.

Heavy ship with powerful attack and range, strong against warships and fortifications alike.

Fire Hazard — Fire ships are most powerful against any form of heavy ship, so keep these as far away from your roundships at all costs.

On Stranger Tides — Factions fronting the Yellow Sea build roundship-type units earlier than Europeans, which are known for their exorbitant cost in timber but cost far less in metal. The Tower Ship can be built well in the Dark Age, but is succeeded by the stronger War Junk in the Castle Age.